1. Field of the Invention
The present invention relates to an FM (Frequency modulated) detector used in e.g., a satellite broadcast receiving apparatus.
2. Background of the Art
Since FM modulation signals are used for satellite broadcasting, an FM detector is incorporated into a satellite broadcast receiver. A PLL (Phase Locked Loop) type FM demodulator is generally used for such an FM (Frequency Modulated) modulator.
In rainy weather the sensitivity and C/N (receiving Carrier to Noise) ratio of satellite antenna receiving FM modulation signals are often reduced. Under such conditions, since a threshold phenomenon occurs which becomes out of the lock-in range of the PLL, a threshold extension is applied to the FM demodulator to prevent such a threshold phenomenon from occurring.
One way to accomplish this is to vary the bandwidth of a band-pass filter which limits the cut-off frequency of a low-pass filter, e.g., a PLL loop and a bandwidth input to the FM demodulator can be used. However, it is difficult to incorporate such a circuit in an FM satellite broadcast receiver due to the complexity of the structure.